Propagation of Light

Velocity of Light. - Light is a transverse wave motion. It travels through empty space, as well as through such transparent substances as glass, air, and water. Its velocity, which is 186,000 miles per sec, is so great that in 1 sec it would travel more than seven times around the earth at the equator. Light travels from the sun to the earth in a little over 8 min, but it requires 4 years for light to travel from the nearest star to the earth. If the North Star were obliterated, the earth would continue to receive light from it for about 44 years.

Frequency and Wave Length. - The relation between frequency, velocity, and wave length is the same for light waves as it is for sound waves. Waves of yellow light have been found to have a wave length equal to about 0.000059 cm. The wave length of light is often expressed in angstrom units. One angstrom unit = 10^-8 cm.

Rectilinear Propagation of Light. - Under ordinary circumstances light travels in straight lines and does not appreciably bend around objects. That light travels in straight lines may be shown by placing a candle or other source of light behind a screen having in it a small hole (Figure 36). In front of this screen AB are placed two screens CD and OF, each with a small hole at the center. When these screens are so adjusted that the eye E can see the source of light S distinctly, it will be found that the straight line joining S and E passes through the holes in the screens. This shows that light from S to E comes in a straight line.

Figure 36 - Rectilinear propagation of light

Sources of Light. - The sun is the chief source of light and heat, but there are many artificial sources. Any body when heated to a sufficient high temperature becomes a source of light.

As the temperature of a body is raised, the body emits invisible radiations. When it becomes red-hot, visible radiations begin to be emit­ted. The higher the temperature, the greater is the amount of both heat and light waves that are emitted, but the percentage of visible radiations becomes larger and larger as the temperature of the source of radiations is increased. For this reason, the modern tungsten lamp is much more efficient than the old carbon incandescent lamp. Tungsten has a very high melting point, and when it is surrounded by nitrogen or when it is in a vacuum, it can be heated to a high temperature and its efficiency thus made large.

3.1.2 Read, translate and retell. Reflection and Refraction of Light

Laws of Reflection. - When a beam of light, travelling in a homogeneous medium, comes to a second medium, some of the light is reflected. At a polished or silvered surface, nearly all the light is reflected. At the surface of clear glass, only a small part of it is reflected. The greater part of it enters the glass and passes through. In Fig, 37, let AB represent the reflecting surface, MP the perpendicular or normal to this surface, OP the incident ray, and PN the reflected ray. The angle OPM between the incident ray and the normal to the surface is called the angle of incidence. The angle MPN between the reflected ray and the normal to the surface is called the angle of reflection. Reflection at such a surface occurs according to the following two laws:

1. First law of Reflection. The incident ray, the reflected ray, and the normal to the surface lie in the same plane.

2. Second law of Reflection. The angle of incidence is equal to the angle of reflection.

Figure 37 - Reflection of light from a plane mirror. The angle

of incidence is equal to the angle of reflection

Refraction. - Experiments have shown that light travels with the greatest speed in a vacuum and that it travels with different speeds in different mediums. When it passes obliquely from one medium to another in which it has a different velocity, there occurs a change in the direction of propagation of the light. This bending of the ray of light when passing from one medium to another is known as refraction.

Refraction can be illustrated by taking a cup which is opaque (Figure 38) and placing a coin on the bottom of it at the point B, so that the far edge of the coin can just be seen when the eye is at E. If now, without moving the eye, water is poured into the cup, the coin will come completely into view. The ray BA as it leaves the water is bent away from the normal NA. Other rays are bent in a similar manner, and an image оf the coin is formed at C, so that the depth of the coin below the surface of the water seems to have been lessened. Here it is seen that rays coming from the water to the air are bent away from the normal. The rays are always bent away from the normal when they enter a medium in which their velocity is greater than it was in the medium from which they came.

Figure 38 - Refraction of light. The rays bend away

from the normal on leaving the water

Refraction through a Prism. - A wedge-shaped portion of a refract­ing medium bounded by two plane surfaces is called a prism. If the medium of which the prism is composed is optically denser than the surrounding medium, a ray of light incident on one of the faces will be bent toward the normal to the face on entering the prism. On emerg­ing from the opposite face, the ray will be going from a denser to a rarer medium and will be bent away from the normal at that face. The angle through which the ray has been deflected in passing through the prism is called the angle of deviation. When the angle at which the ray enters one face is equal to the angle at which it leaves the opposite face, the angle of devia­tion has its least value and is known as the angle of minimum deviation.

Critical Angle. - When a ray of light passes from a dense medium such as water to a rarer medium such as air, it is bent away from the normal so that the angle of refraction is greater than the angle of inci­dence. If the angle of incidence is made larger and larger, the angle of refraction will also become larger and larger and will always be greater than the corresponding angle of incidence. When the angle of incidence is increased sufficiently, the angle of refraction becomes 90 deg, and the refracted ray travels along the surface of separation between the two mediums. That angle of incidence for which the angle of re­fraction is 90 deg is called the critical angle.